In an arc discharge lamp, there is a small probability that the arc tube will burst during lamp operation. The cause of such a burst is unknown and unpredictable. There is no known way to eliminate the possibility of this type of burst. Although an arc tube burst occurs infrequently, there may be a safety hazard to persons or property in the immediate surroundings as a result of such a burst. Consequently, protective measures must be taken.
One protective measure is to require that an arc discharge lamp be operated within a fixture or enclosure capable of withstanding a lamp burst. This procedure has several drawbacks including the facts that a protective fixture may not always be available and removal of a burst lamp from a protective fixture may present a safety concern of a different nature.
A preferred protective measure is to provide a lamp capable of self containment. There are numerous devices which may be incorporated in an arc discharge lamp for the purpose of containing within the outer envelope the forces and shards emanating from an arc tube burst. By so containing the burst, the potential hazard to persons and property is virtually eliminated even when a protective enclosure is not employed.
Examples of typical self-containment devices are as follows. A glass shield may be placed within the outer envelope surrounding the arc tube. The shield may be reinforced with a metal or fibrous mesh. The wall thickness of the outer envelope may be increased. A thick-walled outer envelope may be employed in combination with a shield. A protective coating may be applied to the outer envelope, shield, or both. These containment devices, while necessary and effective, nevertheless increase the cost of a lamp and may adversely affect lamp performance to some degree.
While a protective enclosure or containment device may effectively negate the external effects of an arc tube burst, neither attacks the basic problem of the cause of the burst. Needless to say, there would be no need for either safeguard if the cause of the burst were removed. It would be a substantial advancement of the art if an arc tube were provided for which the possibility of a burst during normal lamp operation were virtually eliminated.
The following prior art is believed to be pertinent to the invention. In U.S. Pat. No. 3,858,078, issued Dec. 31, 1974, to Koury, there is shown an arched arc tube for horizontal operation having a starting or auxiliary electrode in one end of the arc tube. In Gungle et al., U.S. Pat. No. 4,056,751, issued Nov. 1, 1977, there is disclosed an arched arc tube for horizontal operation wherein the electrodes are positioned below the central axis of the arc tube. Karlotski et al., U.S. Pat. No. 4,498,027, issued Feb. 5, 1985, discloses an arched arc tube for horizontal operation wherein the lower wall has lesser curvature than the upper wall such that there is a straight line of sight between the electrodes unimpeded by the curvature of the lower wall. Koza, in U.S. Pat. No. 4,142,122, issued Feb. 27, 1979, describes a lamp intended to be mounted vertically which has an arched arc tube for horizontal operation.
A straight arc tube for horizontal operation is shown in Howles et al., U.S. Pat. No. 4,001,623, issued Jan. 4, 1977. The arc tube of Howles et al. has a cylindrical body and electrodes positioned below the central axis of the operationally positioned arc tube. U.S. Pat. No. 4,232,243, issued Nov. 4, 1980, is another example of an straight cylindrical arc tube intended for horizontal operation having offset electrodes.
In U.S. Pat. No. 3,614,508, issued Oct. 19, 1971, to Ito et al., there is disclosed a metal halide discharge lamp having a starting electrode adjacent each main electrode in each end of the arc tube. Each starting electrode of Ito et al. is electrically coupled with the opposite main electrode. Nakamura shows another example of an arc discharge lamp having two auxiliary electrodes with each auxiliary electrode being electrically coupled with the opposite main electrode.
In Wyner et al., U.S. Pat. No. 4,323,812, issued Apr. 6, 1982, there is disclosed an arc discharge lamp having an arc tube with a starting probe adjacent a main electrode in one end thereof wherein the starting probe is electrically isolated at all times from any ohmic contact with any other electrode of the lamp and from any ohmic contact or capacitive contact from associated ballast circuitry.